Iron oxide pigments having an average particle size of less than 0.1 .mu.m are described as transparent iron oxide pigments as they transmit visible light. As an alternative to the particle size, the specific surface area is often used as a measure of the size of the particles. In particular in the case of acicular particles, this measure avoids stating particle sizes in various dimensions. In this manner, powders having BET specific surface areas of over 80 m.sup.2 /g may be described as transparent. Highly transparent pigments are those having specific surface areas of at least 100 m.sup.2 /g.
Transparent iron oxide pigments are principally used in the production of automotive effect lacquers, wood varnishes and for colouring transparent plastics.
Transparent yellow iron oxide pigments are generally synthesised by precipitation of iron(II) hydroxides or carbonates in an acidic or alkaline pH range and subsequent oxidation to Fe(III) oxidehydroxides. The precipitation of yellow .alpha.-FeOOH in an alkaline pH range is described in U.S. Pat. No. 2,558,303 and U.S. Pat. No. 2,558,304.
While production with an alkali excess does indeed give rise to products which are low in salt and are thus more readily dispersible, it does, however, have the disadvantage that many other hydroxides of metal ions, which are present as impurities in iron salt solutions used industrially, are coprecipitated in addition to the iron hydroxides. The precipitation of Mn impurities may in particular be cited. These impurities cause dirty, greenish hues in the yellow pigments and brownish hues in the red pigments obtained from the yellow pigments by calcination. Moreover, depending upon the particular raw material, the total content of metallic impurities may be so high that these pigments may no longer be used, for example, in plastic packaging for foodstuffs.
The production of .alpha.-FeOOH pigments in an acidic pH range has the advantage that the precipitation of foreign metal impurities is avoided, but, on the other hand, has the disadvantage that, even after vigorous washing, the pigments are still heavily contaminated with anions. Since production is generally performed on the basis of iron sulphate solutions, sulphate impurities may in particular be cited. This leads, both in the yellow pigments and the red pigments obtained therefrom by calcination, to impaired dispersibility and thus, with the same dispersion energy input, to poorer transparency in comparison with pigments produced under alkaline conditions.
U.S. Pat. No. 4,758,415 describes a process for the removal of soluble metal cations from iron oxide pigments. In this process, transparent yellow iron oxide pigments produced using an acidic precipitation process are washed with a solution containing a ligand compound, wherein the conductivity of the pigment is very low after treatment. This process cannot, however, remove metal impurities which are firmly incorporated into the pigment, which occur, for example, when the pigments are produced using alkaline precipitation processes. Ammonia is cited as a ligand compound, in addition to many other compounds. The described post-treatment of pigments with ammonia does, however, have various disadvantages. Thus, for example, on toxicity grounds it is hardly possible to wash filter cakes with ammonia on a large industrial scale. Moreover, when filter cakes are washed on a large industrial scale, it cannot be ensured that the entire product cross-section comes into contact with a sufficient quantity of and for a sufficient time with the ligand compound, such that uniform product quality cannot be achieved.
U.S. Pat. No. 4,758,415 furthermore describes pretreatment of yellow pigments produced under acidic conditions before washing with the ligand compounds as described above, by adding NaOH to the production suspension with simultaneous oxidation. This alkalisation of the production slurry does, however, have the disadvantage that many metallic impurities still contained in the suspension from the raw material are still coprecipitated, in a similar manner as in the precipitation process in an alkaline pH range.